Dry developers used for image development by image forming devices employing electrophotographic technology are generally classified into two-component developers consisting of a toner and a carrier made of ferrite powder, iron powder, glass beads, etc., magnetic single-component developers consisting of a toner containing magnetic powder, and non-magnetic single-component developers. Toners used in these developers are mainly made of binder resins and coloring agents, to which other materials are added including waxes that improve low-temperature fusing property onto a recording sheet and releasing property from the fusing member, charge control agents that add polarity (positive or negative electric charge), etc. After these materials are mixed at the prescribed ratios, they are made into toner of powder form by the process of melting, kneading, pulverizing, classifying, etc. Finally, surface treatment is done with external additives, such as silica, titanium oxide, alumina, resin fine particles, etc. for the purpose of controlling fluidity, charging property, cleaning property, storage stability, etc., to obtain the final developer.
Mainstream toner binder resins include styrene-acrylate resins and polyester resins. However, toners using styrene-acrylate resins have low fracture strength and are therefore easy to generate fine powder dust, despite offering good environmental resistance characteristics. On the other hand, toners using polyester resins have high fracture strength and do not generate fine powder dust easily, but their environmental resistance characteristics are poor.
The developers currently available on the market are naturally manufactured with due consideration to human and environmental safety, and pose no problem in their practical use. However, one trend seen in developers of late is the selection of materials that are safer to the human body and environment. It has become particularly essential to select binder resins, which account for a majority of the toner composition, by considering safety and environment from the viewpoints of component monomers, residual solvents, and so on. In this climate, the suitability of cycloolefin copolymer resins as toner binder resins is drawing the attention of late, and toners using cycloolefin copolymer resins are already disclosed in Publications of Unexamined Japanese Patent Application Nos. Hei 9-101631, 2000-284528 and 2000-206732. These resins consist of non-toxic monomers and provide a lower specific gravity and higher intrinsic volume resistance compared with styrene-acrylate resins and polyester resins, thus offering excellent development property and transfer property (transfer efficiency) and enabling more sheets to be printed per unit weight (less toner consumption). Furthermore, cycloolefin copolymer resins provide high facture strength, so they can extend the life of developers. Also, their excellent optical transparency makes these resins suitable for use in full-color toners.
On the other hand, heat-pressure fusing system has become the mainstream technology adopted by fusers that fuse a developer onto recording media, in line with the trend for higher copying speeds. In this method, a transfer paper is passed between a fusing member contacting toner and a pressure roll not contacting toner, and then heat and pressure are applied simultaneously to melt and fuse the toner onto the transfer paper. Many of the fusing members adopting this method are rolls incorporating a heat source, but belts made of heat-resistant film, etc., are also used. On the other hand, the member not contacting toner generally consists of a pressure roll. To prevent molten toner from adhering to the surface of the fusing member during the toner fusing process, substances providing good releasing property with respect to binder resins such as styrene-acrylate resins and polyesters are selected to form the surface of the fusing member. In particular, excellent releasing property is required for the fusing member contacting toner. Representative examples of these substances offering releasing property are polytetrafluoroethylene (may also be referred to as “PTFE” hereinafter) and silicone rubber. However, these substances offer low heat resistance and therefore the allowable surface temperature of the fusing member cannot be set high or maintaining the surface temperature to a certain level is difficult. As a result, the toner fusing speed cannot be raised and this has been limiting how much the copying speed could be increased.
In addition, toners containing cycloolefin copolymer resins as binder resins tend to cause a so-called “wrapping” phenomenon, in which the toner, even when release agents are added, prevents the printed surface from separating from the fusing member and consequently causes the paper to wrap around the fusing member. This occurs when a normal heat-pressure type fuser, where the fusing member surface and pressure roll surface adopt a combination of PTFE/silicone or silicone/silicone (top/reverse sides of the printed paper), is used. This problem can be improved by reducing the low molecular weight component of cycloolefin copolymer resins, but this measure also reduces the fusing property at low temperature and therefore does not provide the best solution. For this reason, the problem of wrapping has not been fully resolved. Additionally, cycloolefin copolymer resins have a very strong compatibility with polypropylenes, polyethylenes and natural waxes used in common toner binder resins such as styrene-acrylate resins and polyester resins, and therefore cannot provide sufficient releasing property from the fusing member even when release agents are added. Furthermore, cycloolefin copolymer resins are strong and thus cause the surface of the fusing member to wear easily.
On the other hand, copiers and printers sold in recent years are using fewer consumable parts to improve maintainability. As a result, the number of members that cannot be replaced by the user is increasing, and the fusing member is one of such members. Parts not replaceable by the user must remain trouble-free for a long period, but fusers are prone to occur problems. Frequent occurrences of the aforementioned wrapping and other problems add burdens to the user and also require replacement of the problem member by a service provider, which results in downtime during daily operating hours.